1. Field of the Invention
The present invention relates to a radio communication terminal, a communication method, and a radio communication system.
2. Description of the Related Art
In 2002, 3G (which is called third-generation in Japan) mobile phone service started. At first, users mainly used applications each handling a packet which includes small-capacity content such as audio and mail. However, by introduction of HSDPA (High Speed Downlink Packet Access) and the like, the usage behaviors of users have shifted to downloading large-sized packets, for example, downloading music files, or viewing videos provided by YouTube (registered trademark).
In this way, with the increase in downloading packet capacity, radio networks have been expanded and also, a service of HSPA (High Speed Packet Access)+, which realizes up to 21 Mbps in terms of download speed, has been started. Further, in about 2010, an LTE (Long Term Evolution) service which employs OFDMA (Orthogonal Frequency Division Multiplexing Access) on the downlink, and in about 2015, a 4G service are scheduled to be started. A world is coming, where a maximum of 1 Gbps in a semifixed environment, and a maximum of 100 Mbps even in a mobile environment can be realized.
However, only a user who is located near a base station can receive the benefit of high communication rate radio environment described above, and, as the user approaches a cell edge, the communication rate decreases basically. Therefore, it is not necessarily easy to acquire a radio communication environment at stably high communication rate in a mobile environment within one radio communication system. Further, when considering that the use of receiving a video streaming service under high speed mobile environment such as inside a train will be increased in the future, there is desired a technique of handover which is capable of maintaining a high communication rate communication environment over multiple radio communication systems.
In order to simplify a description, a concept of handover which omits a concept of sector is shown in FIG. 7. For example, in the case where a W-CDMA (Wideband Code Division Multiple Access) type mobile terminal moves while talking thereon from a cell A_1 to a cell A_2 which receive W-CDMA service, the mobile terminal transmits/receives a signal to/from two base station at the same time in a area near a cell edge, and soft handover is performed in a manner that the mobile terminal moves into the cell A_2 without breaking the connection.
Further, as an extension function of W-CDMA, a service such as HSPA which provides a high rate packet communication environment is started, and an environment capable of performing large-capacity packet communication is being developed. Note that a function called AMC (Adaptive Modulation and Coding) used in HSPA is for increasing communication rate with respect to a user with better communication state by performing multiple transmission with multilevel modulation, high encoding ratio, and multiple orthogonal codes. Accordingly, it is difficult for a user to receive a high communication rate service in an area such as a cell edge where the reception state is not necessarily good.
Therefore, it is difficult for the user to move from the cell A_1 to the cell A_2 while continuing to receive a high communication rate service and to perform handover without being interrupted like in the case of audio communication. That is, it can be said that the present communication environment is an environment that can provide a service at a high communication rate under the case where the user does not move a lot, or is an environment that can provide a download service at an intermittently high communication rate even while the user is moving, such as web browsing.
FIG. 8 is a flowchart showing a procedure of general handover processing. First, when communication is started or handover is completed (Step 201), a radio communication terminal monitors the reception quality of a cell which performs communication (hereinafter, serving cell) at regular intervals (Step 202). When the reception quality of the serving cell exceeds a threshold (“Yes” in Step 206), only regular monitoring of the serving cell is continued. If the reception quality of the serving cell becomes equal to or lower than the threshold (“No” in Step 206), cell search processing for specifying a candidate neighbor cell is activated (Step 207).
When creation of a measurement report and report to a network side is performed (Step 208) and a candidate is specified from among neighbor cells in the network side (Step 209), a reception quality of the specified cell candidate is measured for a certain period of time (Step 210). When the reception quality of the serving cell is not enhanced and the reception quality of the specified cell candidate reaches a level that is determined by the base station side to perform handover (“Yes” in Step 211), an instruction to perform handover is issued from the base station side to the mobile terminal, and the handover is executed (Step 212).
Further, since the services such as LTE and 4G are planned in the future as described above, there will be developed radio communication paths which provide multiple high communication rates. In that case, as shown in FIG. 9, it is assumed that, at a cell edge of a cell A of a radio communication system, there may be built a cell B of a radio communication system which is different from the radio communication system of the cell A. Under such radio communication environment, there will be no problem in performing handover between the cell A_1 and the cell A_2 in the same manner as shown in FIG. 7 for services with low communication rate such as audio communication.
On the other hand, as for a high communication rate service, in the case where the radio communication terminal moves from the cell A_1 to the cell A_2, the following processing becomes possible: within the cell A_1, before reaching the cell edge, hard handover is performed to a cell B which provides a radio communication system which is different from the radio communication system of the cell A, to thereby maintain a high communication rate; and next, hard handover is performed to the cell A_2. In addition, more stable service can be provided by recognizing a present cell congestion state by the radio communication terminal, and by obtaining means to bring forward the timing of the handover depending on the congestion state. Accordingly, what is important is that the way in which a method of handover is changed depending on a service, and, in addition, a congestion degree of the present cell.
Further, for example, there is disclosed a technology in which an incoming call rate is enhanced even in the case where electric field intensity currently being received is in a weak electric field state to an extent that it cannot normally receive a paging channel (for example, refer to JP-A-2007-53437). According to the technology, in a standby mode, a cellular phone terminal measures an electric field intensity of a currently receiving cell and an electric field intensity of a monitoring cell, and in the case where it is determined that the electric field intensity of the currently receiving cell is weaker than a threshold, the cellular phone terminal determines that the incoming signal strength of the currently receiving cell is not an environment which can normally receive the paging channel. The cellular phone terminal determines whether a cell having an electric field intensity stronger than the electric field intensity of the currently receiving cell is present. In the case where it is determined that a cell having an electric field intensity stronger than the electric field intensity of the currently receiving cell is present, the cellular phone terminal performs cell-reselection operation, in which a cell for receiving the paging channel is switched from the currently receiving cell to the monitoring cell that is determined to have high reception strength.